There's no precise formula for toe-in, but it helps to understand what happens when the brake shoes touch a moving rim.

As the brakes engage the moving rim pushes the brake shoe forward imparting a twisting force to the brake arm and bringing the toe out slightly. The rear corner now touches alone and squeals in a way similar that new chalk sounds on a blackboard. The squeal usually goes away with increased brake pressure which forces the show down flat.

With wear the rear corner breaks down and the noise goes away, but to get the most even wear from your shoes you want to toe them in slightly to offset this twisting so they lay flat on a moving vs a stationary rim. The correct amount of toe in depends on the friction property of the shoe and the rigidity of the brake arm. I usually use a piece of cardboard (about 1/32") trapped under the rear of the shoe to establish toe-in when I mount new shoes.

BTW- a better way to reduce or eliminate squeal is to use a file to break down the rear corner of the shoe slightly before installing. Also note that toe-in isn't critical since shoes will naturally wear more in the rear until they establish they're own toe-in anyway. But excess toe-in should be avoided because it uses up avalaible brake travel and makes the brakes feel spongy.